Create a custom item template:
Create a .cs file with the header the way you want it.
Go to File -> Eport Template
Choose Item Template
select the project that has your file
click Next
Select your file
Next
Select References
Next
Set name, description, images
You can use any image but you may want to pick one out of the Visual Studio image gallery
Choose Automatically import template into visual studio
Next
Now when you right-click a project and select New Item your template will be in the list with all the other templates. You may need to search for it. You will also have a zip file that you can give to others for importing into their Visual Studio.
You can get more elaborate if you need to prompt a user for information but this requires a .vsix project.
As far as what to do about pre-existing classes - you will either have to open them all and paste the header in or write some code that goes through the folders of your solution, somehow identifies the .cs files you want (by namespace maybe?) and append the header. The .cs files are just text after all.